Gas turbine engines generally include a compressor for compressing an incoming air stream. The air stream is mixed with fuel and ignited in a combustor for generating hot combustion gases. The combustion gases in turn flow to a turbine. The turbine extracts energy from the combustion gases for driving a turbine shaft. The turbine shaft powers the compressor and generally an external load such as an electrical generator.
Exhaust emissions from the hot combustion gases generally are a concern and may be subjected to mandated limits. Certain types of gas turbine engines are designed for low exhaust emissions operation, and, in particular, for low NOx (nitrogen oxides) operation. Low NOx turbine engines generally include a combustor in the form of a number of burner cans circumferentially adjoining each other around the circumference of the engine.
NOx emissions generally are related to the combustion temperature within the combustor. To control the temperature and the NOx emissions output, water can be added to the combustion process via a manifold positioned about the combustor. The amount of water generally is controlled as a function of fuel flow, relative humidity, and ambient temperature.
A load rejection is an instantaneous disconnection of the turbine generator from an electrical grid. The supply of fuel to the turbine therefore must be cutback quickly to prevent an over speed condition. Cutting back the fuel supply also must be done in a controlled manner so as to maintain flame stability. As the amount of water injected for NOx abatement is a function of fuel flow, the water flow to the combustor also must be stopped during a load rejection. Water injection manifold pressure reduction, however, may not always be fast enough to reduce the possibility of a flameout, i.e., the water already in the manifold may continue to flow into the combustor.
Thus is a desire, therefore, for an improved water manifold pressure relief system design that can react quickly to a load rejection. The increased reaction time should limit the possibility of flameout and the associated restart expenses.